THE GEOLOGY OF THE ANSIL DEPOSIT, ROUYN-NORANDA, QUEBEC

Special Volume, Vol. SV 43, No. 1990, 1990

The Ansil deposit is located in the Noranda mining camp in the southern part of the Abitibi greenstone belt. It was discovered in 1981 and production began in 1989 with proven reserves of 1.58 Mt grading 7.2% copper, 0.9% zinc, 26.5 glt of silver and 1.6 glt of gold. It is centered at a depth of 1.3 kilometre and represents one of the deepest ore discoveries ever made from surface. The deposit consists of a single massive sulphide lens located at the contact between the Northwest Rhyolite and the Rusty Ridge Andesite. The lens strikes north-south and dips 50° east. The thickness of the lens may reach up to 35 m and can change very abruptly both along strike and along dip. The maximum thicknesses of the sulphide lens are reached in its upper (western) and at its lower (eastern) edges, thus giving the deposit its present shape of two sulphide domes connected by a thinner central portion. The lower dome is underlain by a thick accumulation of massive magnetite which may reach 10 to 15m in thickness. The upper and lower domes are underlain by two distinct pipe-shaped zones of stringer and disseminated sulphides which merge with a larger stringer zone at depth. The Ansil deposit displays strong relationships between its morphology and the distribution of metals. Hydrothermal alteration consists of chloritization and sericitization. Chlorite alteration is closely associated with stringer sulphide mineralization while sericite is developed in diffuse zones bordering the chloritic zones. Unlike other similar deposits, the Ansil deposit is characterized by very extensive alteration and sulphide mineralization in the hangingwall of the massive sulphides. Several features of the deposit are consistent with an origin from a submarine hydrothermal system which was particularly long-lived, and which thus promoted the build-up of very high fluid temperatures with extensive replacement (and enrichment) of pre-existing sulphides after their original deposition onto the seafloor. Le gisement Ansil se trouve au centre du camp minier de Noranda, lui-meme localise dans la partie sud de la ceinture de l'Abitibi. Decouvert en 1981, il a ete mis en production en juillet 1989 alors que les reserves prouvees s' elevaient a 1.58 Mt a 7.2% de cuivre, 0.9% de zinc, 26.5 glt d' argent et 1.6 glt d' or. Le coeur de ce gisement est situe a 1 300m de profondeur et sa decouverte est l'une des plus profondes jamais faites a partir de la surface. Le gisement Ansi/ comprend une seule lentille de sulfures massifs localisee au contact de Ia rhyolite Northwest et de l'andesite Rusty Ridge. Cette lentille, orientee nord-sud et inclinee de 50° vers l'est, montre une epaisseur pouvant atteindre 30m, qui peutvarier brutalement le long de la direction et enpendage. Les epaisseurs maximales se retrouvent aux deux extremites ( superieure et inferieure) de la lentille,lui confer ant une forme bilobee a deux domes separes par une zone midiane plus mince. Le dome de la partie inferieure est caracterise par une epaisse accumulation de magnetite massive, sous-jacente aux sulfures, pouvant atteindre 10 a 15 m d'epaisseur. Le gisement Ansil montre une relation directe entre sa morphologie et la distribution des metaux. L'alteration hydrothermale associee au gisement consiste principalement en chloritisation et serecitisation. La chlorite est intimement liee aux zones riches en veinules de sulfures, situees de part et d' autre des sulfures massifs.La sericite se retrouve dans des zones diffuses qui enveloppent les zones chlorite uses. Contrairement aux autres gisements de Ia region, celui d'Ansil est caracterise par la presence, au-dessus des sulfures massifs, d' une grande quantite de sulfures dissemines et de roches alterees. Plusieurs caracteristiques presentees par ce gisement confortent l'hypothese de sa mise en place a partir d'un systeme hydrothermal sous-marin fonctionnant sur une longue periode et permettant, ainsi, Ia formation de fluides a haute temperature qui vont favoriser le remplacement et l'enrichissement des sulfures apres leur accumulation sur le fond marin.

Keywords: Ansil deposit, Noranda Mining Group, Rouyn-Noranda, Abitibi gemstone belt, Lens, metal distribution, Gisement Ansil, Camp Minier Noranda, Ceinture de l'Abitibi, Lentille, Distribution des metaux.